1. Field of the Invention
The present invention relates to a vehicular brushless AC generator and an exciting-coil bobbin unit therefore mounted on motor vehicles such as an automobile, bus and truck.
2. Description of the Related Art
A conventional vehicular brushless AC generator includes an exciting-coil bobbin unit, which is formed of resin with a uniform thickness so that the bobbin will not be easily deformed.
Patent Document 1 disclosed a structure of joining a yoke with a plate is employed that decreases the inner diameter of a bobbin while ensuring strength of supporting the plate, and in order to obtain a field-coil bobbin unit for a brushless alternator that can reduce the material for the field coil, thereby enabling cost down while ensuring vibration resistance, an annular step portion is formed on an end side of the inner circumferential surface of a thick ring-shaped second yoke, the other axial end of a cylindrical portion of the plate is engaged with the step portion as radially overlapping each other, and the cylindrical portion of the plate is joined to the second yoke by welding in the region of the cylindrical portion radially overlapping the step portion.
Moreover in Patent Document 2, in order to provide a brushless alternator in which an output current is enhanced by configuring an optimized magnetic circuit, the alternator includes: a step portion provided on an inner circumferential surface of an axial end portion of a yoke; a plate having a cylindrical portion fixed to this step portion and made of magnetic material; a bobbin disposed surrounding the radial outer circumferential surface of the cylindrical portion of the plate so that a field coil is wound thereon; and a stator provided outside a rotor retaining an air gap, in which magnetic flux produced by the field coil propagates; wherein the cylindrical portion of the plate fixed to the step portion provided on the inner circumferential surface of the yoke is formed to have a radial thickness of greater than 1 mm but smaller than 2 mm.
Patent Document 1: Japanese Laid-Open Patent Publication No. 2005-80440
Patent Document 2: Japanese Laid-Open Patent Publication No. 2007-37233
However, in the conventional vehicular brushless AC generator, the thickness of the bobbin unit made of resin has greatly affected its output characteristics, and if the thickness of the bobbin is reduced, the creeping distance to the yoke with which the bobbin is engaged will be shortened, causing a concern in that the electrolytic corrosion resistance of the exciting coil would be deteriorated between the yoke at low potential and the exciting coil at high potential.
Moreover, another concern has been that bobbin thin portions would be deformed when the bobbin is molded or an exciting coil is wound thereon in production process of the exciting-coil unit, leading to loss of productivity.